fix(msbuild): Stride.AssetCompiler.targets - use Path.Combine for paths

[VaclavElias]

PR Classification

Bug fix to ensure correct path handling in asset build processes.

PR Summary

Replaced string concatenation with System.IO.Path.Combine for StrideCompileAssetBuildPath and StrideCompileAssetUpToDateCheckFileBase in Stride.AssetCompiler.targets, ensuring intermediate output paths are handled correctly for both normal projects and file-based apps (dotnet run app.cs), whose obj/bin paths are already rooted under the SDK's temp cache.

• Stride.AssetCompiler.targets: use Path.Combine instead of concatenation for StrideCompileAssetBuildPath and StrideCompileAssetUpToDateCheckFileBase; added explanatory comments

Testing notes

This PR fixes StrideCompileAssetBuildPath and StrideCompileAssetUpToDateCheckFileBase to use
Path.Combine instead of string concatenation, so absolute intermediate paths (e.g. file-based
apps via dotnet run app.cs, whose obj/bin live under the SDK's temp cache) are handled correctly
instead of producing a malformed path.

Verified on Windows. Path.Combine/Path.IsPathRooted rootedness semantics can differ slightly
across platforms, so it would be good to double check normal + file-based app builds on
Linux/macOS, if a maintainer has the setup handy.

[VaclavElias]

References: https://learn.microsoft.com/en-us/dotnet/core/sdk/file-based-apps

Additional context, this change will allow to run Stride (code-only) from any C# file e.g. Program.cs without a project file csproj like the example below.

Without this PR, we have to add also these two lines

#:property BaseIntermediateOutputPath=obj\
#:property OutputPath=bin\

#:package Stride.CommunityToolkit.Bepu@1.0.0-dev
#:package Stride.CommunityToolkit.Skyboxes@1.0.0-dev
#:package Stride.CommunityToolkit.Windows@1.0.0-dev

using Stride.CommunityToolkit.Bepu;
using Stride.CommunityToolkit.Engine;
using Stride.CommunityToolkit.Rendering.ProceduralModels;
using Stride.CommunityToolkit.Skyboxes;
using Stride.Core.Mathematics;
using Stride.Engine;

using var game = new Game();

game.Run(start: Start);

void Start(Scene rootScene)
{
    game.SetupBase3DScene();
    game.AddSkybox();

    var entity = game.Create3DPrimitive(PrimitiveModelType.Capsule);
    entity.Transform.Position = new Vector3(0, 8, 0);
    entity.Scene = rootScene;
}

[xen2]

Let's use this instead:
$([MSBuild]::NormalizePath('$(ProjectDir)', '$(BaseIntermediateOutputPath)', 'stride\assetbuild\data'))
It would match other use (i.e. line 65) and it applies GetFullPath which resolves ..\ from the resulting path and OK with missing trailing slash).